In medical imaging, different imaging modalities may be used to provide clinicians with complementary information. Complementary information may be information that is present in one acquisition but is not present in a different acquisition. Different sequences (for example, T1-weighted and T2-weighted sequences in magnetic resonance imaging) or different acquisition techniques (for example, contrast and non-contrast) may also be used to provide complementary information.
Examples of complementary information may include different physical parameters, anatomical information versus functional information, flow or perfusion, or soft tissue versus hard tissue contrast.
A given modality, sequence or acquisition technique may provide information that it is not possible to obtain using a different modality, sequence or acquisition technique.
For example, a genuine T2-weighted magnetic resonance (MR) image cannot be obtained from a T1-weighted MR image, because there is not a monotonic relationship between the T1 and T2 relaxation times. An MR study may include the acquisition of both T1-weighted data and T2-weighted data, in order to obtain information that is only available in the T1-weighted data, and complementary information that is only available in the T2-weighted data.
Similar considerations may apply to data acquisition using more than one modality. For example, both CT (computed tomography) and MR data may be acquired in order to obtain hard tissue information from the CT data and complementary soft tissue information from the MR data.